Climate of California - Glossary

Glossary

The following are terms used to describe local or regional weather events.

• Santa Ana winds: Santa Anas are hot, high winds that blow from the eastern mountains and deserts towards coastal Southern California, usually in the spring and fall. They are the result of air pressure buildup between the Sierra Nevada and the Rocky Mountains. This air mass spills out, is pulled by gravity, and circulates clockwise around the high pressure area, bringing winds from the east and northeast; as the air descends in elevation, it heats up and the humidity plummets to less than 15%. It is often said that the air is heated and dried as it passes through the nearby deserts, but according to meteorologists this is a popular misconception (it's actually due to adiabatic compression as the air flows from the higher elevations down to sea level); during Santa Ana conditions, it is typically hotter along the coast than in the deserts. As the Santa Ana winds are channeled through the mountain passes they can approach hurricane force. The combination of speed, heat, and dryness turns the chaparral into explosive fuel for the infamous wildfires the region is known for.
• Pineapple Express: A complex combination of events, bringing high levels of torrential precipitation to California. The Pineapple Express occurs when the jet stream dips across California, and warm, humid air from the mid-Pacific (from the vicinity of Hawaii–hence, the name) is drawn towards California. The resulting combination can cause extraordinarily heavy rains. A January 2005 Pineapple Express dropped nearly 25 inches (63.5 cm) of rain in Santa Barbara County.
• Tule fog: A thick ground fog that settles in the San Joaquin Valley and Sacramento Valley areas of the California Central Valley. Tule fog forms during the mid fall, winter to early spring after the first significant rainfall. This phenomenon is named after the tule grass wetlands of the Central Valley. Tule fog can extend from Bakersfield to Chico. Accidents caused by the tule fog are the leading cause of weather-related deaths in California; visibility is usually less than an eighth of a mile (about 600 feet or 183–200 m), but can be less than 10 feet (3 m).
• May Gray/June Gloom: A characteristic weather pattern of late spring (May and June) in which a combination of inland heat, off-shore cool water, and prevailing wind patterns bring foggy and overcast weather to coastal regions. From Point Conception northwards the gloom continues until early autumn. (See also: Catalina eddy)
• Marine layer: The cool, moist layer of air coming in from the ocean which typically includes fog. A standard weather phenomenon along the northern and central California coast from late spring to early fall.
• "Earthquake weather:" Any unseasonal, uncomfortable weather, typically hot and more humid than usual, and often associated with high and mid-level clouds, is spoken of (usually jokingly) as "earthquake weather."